1. Field of the Invention
The present invention relates to filters, and more specifically to a method and apparatus for adjusting the trans-conductance of a filter.
2. Related Art
A filter generally refers to a component which allows/passes a range of frequencies and rejects all other frequencies from an input signal. For example, a low pass filter allows all low frequencies below a cut off frequency (determined by filter components) and rejects all the high frequencies above cut off frequency.
Filters are often implemented within integrated circuits, and are thus characterized by trans-conductance. Trans-conductance generally provides a measure of the degree of conductivity of a filter and equals the reciprocal of resistance. Filter characteristics such as frequency response and amplification factor are determined by (among other factors) the trans-conductance value as is well known in the relevant arts.
One problem often encountered with filters is that the actual trans-conductance value is at variance with a desired value. The variance can be for reasons such as imperfections in manufacturing technologies and material, changes in operating conditions (e.g., ambient temperature), etc. Accordingly, it is desirable to adjust the trans-conductance value of a filter during operation.
A tuning circuit provided according to an aspect of the present invention can be used to tune a component (e.g., filter) to a desired trans-conductance value. The tuning circuit may contain a trans-conductor circuit generating a present signal representing a present trans-conductance of the trans-conductor circuit. The trans-conductance circuit operates to provide a similar transfer function as the component. A reference circuit may generate a reference signal representing the desired trans-conductance value.
An integrating capacitor may receive both the present signal and the reference signal in opposite directions, and be charged to a voltage level representing a difference of the present signal and the reference signal. An amplifier may amplify the voltage level to generate an analog signal. The analog signal may then be used to change the trans-conductance of the trans-conductor circuit and the component.
Due to the operation of the integrating capacitor, the tuning circuit may be implemented using only a single active element (i.e., amplifier). The amplifier can be implemented in an open loop configuration to minimize power consumption. In an embodiment, the amplifier is implemented as a single stage CMOS amplifier.
In an embodiment, the reference signal is generated using a switched capacitor circuit. The tuning circuit may further contain a passive circuit (i.e., containing only passive elements) to eliminate any ripple generated in a voltage signal generated by the switched capacitor circuit.
In an embodiment, the passive circuit contains a first resistor connected in parallel to the integrating capacitor, the first resistor and the integrating capacitor being connected between a first node and a second node. The passive circuit may also contain a second resistor connected between the non-inverting input terminal of the amplifier and the first node. A third resistor may be connected between the inverting input terminal (of the amplifier) and the second node.
More passive components may be provided to eliminate (reduce) any additional ripples present at the output of the amplifier. In an embodiment, a second capacitor, a fifth resistor and a third capacitor may be provided. The second capacitor is connected between the output terminal of the amplifier and a ground, the fifth resistor is connected between the output terminal of the amplifier and a third node, and the third capacitor is connected between the third node and the ground.
In an embodiment, the output of the amplifier may not be stable due to the two poles respectively introduced by the integrating capacitor and the amplifier. Accordingly, the tuning circuit may be provided with a fourth resistor and a first capacitor connected in series between the output terminal and the ground. The fourth resistor and the first capacitor operate to remove the effect of one of the poles by introducing a zero, thereby lending stability to the output signal.
The tune component (e.g., filter) may contain multiple trans-conductor stages. The output of the amplifier may be examined to determine the specific stages to activate and de-activate. A digital bits generator may generate digital bits specifying which ones of the stages to be activated and de-activated.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.